Maybe I misunderstood, when you said he has not been ridden for a while and now you have concerns of Saddle fit, I can only guess it may be a lack of muscle tone, could be the saddle never fit perfectly to start with.
If tone is the cause, there is no formula to say how much, how long.
This Saddle sits down in front a little, and the top of the bar is making a touch more contact than the bottom, this indicates the tree may be too wide.
It is not a lot, therefore the possibility of him needing more toning up.
FYI, a semi-qh 6.5" gullet has approx a 86 degree angle, a reg-qh 6.75" gullet has approx a 89 degree angle and a full-qh 7" gullet has approx a 92
These different are not a lot and sometimes one can get away with one size different, since many saddles come in Medium (semi-qh) and Wide (full-QH) this is impossible on many Saddles, not sure what tree your saddle has in it, so cant say with certainty how far off in fit you are without actually measuring the Horse.
By the picture of your Horse, I do not see the need for a tree with more flair in the bar, I do not see big shoulders, but could be the angle of the camera also, so he may need a tree with more flair than that one.
Have you played with the saddle with no pad to see where the natural pocket of this Horse is, someone mentioned that in another thread, every Horse is a little different.
May all your Trails be happy and safe ones